Photography News | Issue 40 | absolutephoto.com Technique 63 Camera School PART 5 Here we lift the lid on all things camera related, showing how to get better results from your CSC or DSLR and providing all the info you don’t find in the manual. So, stick with us and you’ll soon be wielding your camera like a pro. This month, how the aperture setting you use influences the depth-of-field
Words & pictures by Kingsley Singleton
So far in Camera School we’ve examined the effects of aperture on exposure, showing how the size of the aperture affects the amount of light recorded and therefore the brightness of the image. But there’s a secondary effect of aperture, which is of just as much interest to photographers: it’s called depth-of-field. What is depth-of-field? Depth-of-field is a measurement of how much of the image is considered ‘acceptably sharp’. This depth-of-field might be a few millimetres or many hundred of metres, and it’s governed by several factors, including the size of the aperture, the size of the sensor, the focal length you’re shooting at, and howclose to the camera you set the focus. Acceptable sharpness? Let’s go back to that term ‘acceptably sharp’ for a moment, because it’s important to understand that there’s no hard line between in focus and out of focus. Only the point at which you focus is truly sharp, and there is a gradual defocusing away from that plane, even if those areas appear sharp to your eyes. As areas become defocused, points of light spread and are called circles of confusion. What’s considered ‘acceptably sharp’ is
based on how big these circles of confusion are. Some will appear to perfectly sharp to our eyes, and some won’t; it’s when they begin to affect this perception of sharpness that the depth-of-field ends. Traditionally, the circle of confusion only needs to be 0.001in (0.025mm) in size for that area of the image to lose acceptable sharpness. But acceptable sharpness is also based on the viewer’s eyesight, viewing conditions, the size, the resolution of the image, and more. The important thing to keep in mind, is that the sharper an image looks from the nearest object to the furthest, the greater the depth-of-field can be said to be. It’s also worth remembering that, generally, the depth-of- field extends further behind the point of focus than it does in front of it (towards the camera), and this becomes more pronounced the further away you focus. Aperture and depth-of-field So, how does the aperture setting affect the depth-of-field? Broadly, the larger the aperture (lower f/number), the less depth-of-field there will be. And as you close the aperture, depth- of-field increases. So, when shooting with the same lens and focusing in the same place, if you shoot at a low f/number like f/2.8 or f/4,
you’ll only generate a small depth-of-field and lots of the scene will be blurred; if you shoot with a high f/number like f/16 or f/22, most of the scene will appear to be sharp. How to control depth-of-field Because depth-of-field is one of the most important creative decisions you can make in an image, it makes sense to take full control over it. For this reason, it’s best to shoot in aperture-priority mode (A or Av on the Mode dial), wherein you’ll be able to set the f/number in respect of the amount of sharpness you want, with the shutter speed compensating for it to create a good exposure. You can also control aperture in manual mode, wherein you’ll need to set the shutter speed, too. On most digital cameras aperture is controlled using one of the command dials, but some, like Fujifilm’s X-series, feature traditional-style aperture rings around the lens itself. The aperture setting will be displayed on screen or within the viewfinder, and if you’re using live view mode, you may get a preview of the sharpness of the scene. Most DSLRs also still include a depth-of-field preview button that does the same thing via the viewfinder, although the viewfinder will dim as the amount of light is reduced.
Above Once you’re in aperture-priority mode (A or Av), you’ll have full control of the aperture, and therefore the depth-of-field. By altering depth-of-field you can control howmuch of the scene is in focus (see examples below), which is one of the most creative tools you can use in photography.
NEXTMONTH How the focal length and where you focus affects depth-of-field.
Enlargement at f/4
Full image at f/4
Enlargement at f/8
Enlargement at f/16
Powered by FlippingBook